The present invention relates generally to quartz crystal piezoelectric effect resonators, and particularly to resonators employing a stress compensated ("SC") cut quartz crystal element and methods of operating them.
Recent research has revealed that resonators employing SC cut quartz crystals elements are less affected by a variety of thermally and physically induced stresses than resonators employing other quartz crystal cuts such as an AT cut, for example. See for example, A. Warner, B. Goldfrank, M. Meirs and M. Rosenfeld, "Low `g` Sensitivity Crystal Units and Their Testing"; Proc. 33rd Ann. Freq. Control Symp., USAERADCOM, Ft. Monmouth, N.J. 07703, June 1, 1979, pp 306-311; B. Goldfrank and A. Warner, "Further Developments on `SC` Cut Crystals", Proc. 34th Ann. Freq. Control Symp., USAERADCOM, Ft. Monmouth, N.J. 07703, May, 1980, pp 183-186; B. Goldfrank, J. Ho and A. Warner, "Update of SC Cut Crystal Resonator Technology"; Proc. 35th Ann. Freq. Control Symp., USAERADCOM, Ft. Monmouth, N.J. 07703, May 27, 1981, pp 92-97; and B. Goldfrank, J. Tsaclas and A. Warner, "Further Deveopments In SC Cut Crystal Resonator Technology", Proc. 36th Ann. Freq. Control Symp., USAERADCOM, Ft. Monmouth, N.J. 07703, June 2, 1982, pp 208-221. The disclosures of the foregoing documents are incorporated herein by reference.
Heretofore, resonators employing an SC cut quartz crystal element were operated with an electric field exciting the crystal directed normal to a major surface of the crystal element, i.e. along the thickness of the element. Such excitatio has been referred to as "thickness excitation" or "thickness field excitation", and a field directed normal to a major surface of the element can be referred to as a "thickness excitation field" or simply a "thickness field". Since an SC cut quartz crystal element is a "doubly rotated Y cut", an electric field direction normal to the surface of the crystal element has a component in the X as well as the Y direction of the crystal lattice. As a result, both the B mode of vibration (fast shear mode) and the C mode of vibration are strongly coupled, with the frequency of the B mode being within about 9% of that of the C mode. Typically, the C mode of vibration is perferable to the B mode because its frequency of vibration is relatively temperature stable as compared to that of the B mode. Therefore, when utilizing a resonator having an SC cut quartz crystal element in a circuit such as an oscillator operated at the frequency of the C mode of vibration, it was necessary to suppress or filter out by means of a sharply tuned trap, for example, the B mode frequency to prevent it from interferring with the C mode frequency.
There are applications, however, in which the frequency of the B mode of vibration of an SC cut quartz crystal resonator can be useful, for example, as an exact temperature indicator and also as a check on the angular orientation of an SC cut quartz blank. (See the article by A. Warner, et al. titled "Low `g` Sensitivity Crystal Units And Their Testing" referred to above).
Piezoelectric elements used as resonators are frequently excited by a thickness electric field which is typically generated by placing an electrode on each of the two major surfaces of the piezoelectric element opposite each other. Piezoelectric elements used in resonators have also been excited with an electric field directed parallel to a major surface of the piezoelectric element. Such parallel excitation has been referred to as "parallel field excitation" and more recently as "lateral-field excitation", and a field directed parallel to a major surface of the crystal element can be referred to as a "lateral excitation field" or simply as a "lateral field". Lateral field excitation may be generated by two electrodes disposed on the same major surface of the piezoelectric element or by two electrodes disposed on opposite major surfaces but offset from each other so as to direct the electric field laterally into the piezoelectric element between the two electrodes. Lateral field excitation of piezoelectric crystal elements is disclosed for example in U.S. Pat. No. 3,165,651 of R. H. Bechmann, issued on June 12, 1965, in U.S. Pat. No. 3,202,846 of A. D. Ballato and R. Bechmann, issued on Aug. 24, 1965, and in A. Warner, "Use of Parallel Field Excitation in the Design of Quartz Crystal Units", Proc. 17th Ann. Symp. Freq. Control, 1963, pp 248-266. While resonators employing quartz crystal elements have been operated with lateral fields, they typically utilized an AT cut where the electric field component in the Y direction was only about 50%. Consequently, the impedance was high.
Therefore, while the use of quartz crystal elements in piezoelectric effect resonators provided relatively stable resonant frequencies, there was a problem in suppressing or eliminating frequencies associated with unwanted vibratory modes of the crystal elements.
There is thus a need to selectively control the relative strength of the different vibratory modes of quartz crystal elements utilized as piezoelectric effect resonators.